diff --git a/docs/swarm.md b/docs/swarm.md
index c255d44a..be4897ba 100644
--- a/docs/swarm.md
+++ b/docs/swarm.md
@@ -22,12 +22,23 @@ services.hyperhive = {
   domain   = "pr1ma.example.com";   # machine-addressable DNS domain
   hiveName = "pr1ma";               # human display name (optional)
   swarm.name = "constellat1on";     # shared swarm display name (optional)
+  swarm.domain = "example.com";     # the swarm's DNS domain
 };
 ```
 
-`domain` is required when matrix federation is on (`matrix.enable`);
-it drives `HYPERHIVE_HIVE_DOMAIN` in every container so agents can
-form qualified labels (`iris@pr1ma.example.com`). `hiveName` and
+`domain` is required whenever hyperhive is enabled — eval fails with a
+hint if it is unset. It drives `HYPERHIVE_HIVE_DOMAIN` in every
+container so agents can form qualified labels
+(`iris@pr1ma.example.com`).
+
+You can also *not* write it: with `swarm.domain` and `hiveName` set,
+`domain` defaults to `<hiveName>.<swarm.domain>`, since every hive in a
+swarm occupies its own sub-domain of it. That is a default and not a
+rename — a hive that pins `domain` explicitly keeps exactly the value it
+has today, which is the point: re-rooting where a value *comes from*
+must not reinterpret the values already deployed.
+
+`hiveName` and
 `swarm.name` are purely display — they surface in the dashboard chrome
 header and per-agent system prompts. Federated hives at different
 domains can share a `swarm.name`; that it sits under `swarm` and
diff --git a/nix/host-modules/hive-network.nix b/nix/host-modules/hive-network.nix
index c7f6728e..218a88f8 100644
--- a/nix/host-modules/hive-network.nix
+++ b/nix/host-modules/hive-network.nix
@@ -161,7 +161,10 @@ in
             hive resolver is authoritative for `<hive-domain>` and its
             sub-domains, and agents reach the forge/matrix through the
             gateway by that domain. Pin a hostname
-            (`services.hyperhive.domain = "example.com";`).
+            (`services.hyperhive.domain = "example.com";`), or set
+            `services.hyperhive.swarm.domain` and
+            `services.hyperhive.hiveName` and it is derived for you as
+            `<hiveName>.<swarm.domain>`.
           '';
         }
       ];
diff --git a/nix/host-modules/hyperhive.nix b/nix/host-modules/hyperhive.nix
index 9ea5b35e..bb3f5b16 100644
--- a/nix/host-modules/hyperhive.nix
+++ b/nix/host-modules/hyperhive.nix
@@ -4,8 +4,12 @@
 # ./default.nix aggregator.
 {
   lib,
+  config,
   ...
 }:
+let
+  hiveCfg = config.services.hyperhive;
+in
 {
   # The swarm's display name moved under `swarm` when the swarm-global
   # settings were consolidated; the hive's own name and domain stayed put,
@@ -32,7 +36,19 @@
   # Hive identity (label + domain + display names).
   options.services.hyperhive.domain = lib.mkOption {
     type = lib.types.nullOr lib.types.str;
-    default = null;
+    # Every hive in a swarm lives at its own sub-domain of the swarm's,
+    # so this is derivable rather than something each hive repeats. It
+    # stays a DEFAULT and not a rename: an alias would reinterpret the
+    # domains hives have already deployed, whereas a default only fills
+    # in the ones that never set it. Null (both parts unset) keeps the
+    # existing "required" assertion in hive-network.nix as the single
+    # place this can fail.
+    default =
+      if hiveCfg.swarm.domain != null && hiveCfg.hiveName != null then
+        "${hiveCfg.hiveName}.${hiveCfg.swarm.domain}"
+      else
+        null;
+    defaultText = lib.literalExpression ''"''${hiveName}.''${swarm.domain}", or null when either is unset'';
     example = "darkest.space";
     description = ''
       Canonical host domain for hyperhive subsystems that need a
@@ -46,6 +62,34 @@
       `HYPERHIVE_HIVE_DOMAIN`; consumed by
       `hive-agent::identity::hive_domain()` for `<name>@<domain>`
       qualified labels.
+
+      Defaults to `<hiveName>.<swarm.domain>` when both of those are
+      set, so a hive in a swarm does not restate its own address.
+      Setting this explicitly always wins.
+    '';
+  };
+
+  # Where the swarm lives. Declared beside the hive's own identity
+  # because it is what that identity is derived FROM — every hive in a
+  # swarm is a sub-domain of it. Unlike the renamed options nearby, this
+  # is genuinely new: nothing moved here, so there is no alias.
+  options.services.hyperhive.swarm.domain = lib.mkOption {
+    type = lib.types.nullOr lib.types.str;
+    default = null;
+    example = "darkest.space";
+    description = ''
+      DNS domain of the wider swarm this hive belongs to. Each hive
+      occupies its own sub-domain of it, which is why
+      `services.hyperhive.domain` defaults to
+      `<hiveName>.<swarm.domain>` — set this plus
+      `services.hyperhive.hiveName` and a hive needs no domain of its
+      own.
+
+      A swarm needs this set somewhere to address its hives uniformly;
+      it is left nullable so an existing single-hive deployment that
+      pins `services.hyperhive.domain` directly keeps evaluating
+      untouched. The only thing eval insists on is that the hive ends
+      up with a domain, by either route.
     '';
   };